Main content area

Cosmogenic ³He age estimates of Plio-Pleistocene alluvial-fan surfaces in the Lower Colorado River Corridor, Arizona, USA

Fenton, Cassandra R., Pelletier, Jon D.
Quaternary research 2013 v.79 no.1 pp. 86-99
alluvium, basalt, climate, climate change, mountains, piedmont, terraces, Arizona, Colorado River
Plio-Pleistocene deposits of the Lower Colorado River (LCR) and tributary alluvial fans emanating from the Black Mountains near Golden Shores, Arizona record six cycles of Late Cenozoic aggradation and incision of the LCR and its adjacent alluvial fans. Cosmogenic ³He (³Hec) ages of basalt boulders on fan terraces yield age ranges of: 3.3–2.2Ma, 2.2–1.1Ma, 1.1Ma to 110ka, <350ka, <150ka, and <63ka. T1 and Q1 fans are especially significant, because they overlie Bullhead Alluvium, i.e. the first alluvial deposit of the LCR since its inception ca. 4.2Ma. ³Hec data suggest that the LCR began downcutting into the Bullhead Alluvium as early as 3.3Ma and as late as 2.2Ma. Younger Q2a to Q4 fans very broadly correlate in number and age with alluvial terraces elsewhere in the southwestern USA. Large uncertainties in ³Hec ages preclude a temporal link between the genesis of the Black Mountain fans and specific climate transitions. Fan-terrace morphology and the absence of significant Plio-Quaternary faulting in the area, however, indicate regional, episodic increases in sediment supply, and that climate change has possibly played a role in Late Cenozoic piedmont and valley-floor aggradation in the LCR valley.